By default the AC status indicator does not light up on DSC keypads. There is an option in programming to turn it on if desired. The option is shown in the installation manual that comes with the keypad (not the system) but in brief:

Enter *+8+Installers Code+*
You are now in keypad programming
Select address *077
Turn on options 5 and 6
Now the power on symbol, on the keypad, will illuminate.